When joining one pillar and a second (or an other separate) pillar to each other, for example, in a form that an end face of the second pillar is vertically abutted against a side face of the one pillar, there has been a structure in which the end face of the second pillar is first abutted against a side face of the one pillar, and at the corners of the abutted position, substantially triangular shape of brackets are attached and fixed by fastening screws.
There has been another joint structure, in which a dovetail joint groove has been formed in the one pillar, and a joint body, of which end part elongates away from the dovetail joint groove, is engaged with the dovetail joint groove. Then, the end face of the second pillar is abutted against the side face of the one pillar, and the end part of the joint body is fixed by fastening screws at a predetermined position in the dovetail joint groove of the second pillar.